Machine structural components must satisfy various properties including those relating to fatigue strength, durability life, workability and the like. In particular, fatigue strength is becoming increasingly important with trends toward heavier loads and higher speed together with the requirements of higher performance in industrial machinery and vehicles. Studies are being made to develop a steel having higher fatigue strength durability life, workability, and the like.
In order to improve fatigue strength, a conventional method was propsed wherein an alloying element such as Ni or Mo is added in an appropriate amount to increase the material strength. In some applications, another conventional method was proposed which uses a special smelting method such as VAR or ESR so as to control the solid texture and to reduce the amount of non-metallic inclusions.
However, in the conventional method of simply adding the alloying element, a satisfactory long durability life cannot be obtained in some applications. Again, the latter method is costly and not suitable to mass-production.